40 and 90 sized planes are kinda far apart ... I would say get 2 motors, one for each. They are getting cheap nowadays if you shop at Hobby City. I suspect the 4025-16 on 6S will be best turning a 16x12 prop, which 40 sized plane will have clearance for a 16"? Also the 4025-16 may not gonna be running happily on a 90 sized plane with 8S cause you need to drop the prop a bit as the cell count goes up. Check using something like MotorCalc to get an idea, that would be best.